This invention relates to apparatus for polishing silicon wafers.
Silicon wafers are produced as precursors from which micro-electronic semiconductor components are produced. The wafers are grown for example by deposition of silicon onto a substrate, to produce discs typically 20 cm in diameter, which are split by cleavage parallel to their major surfaces (analogous to the cleavage of slate) to produce two thinner wafers. The resulting wafers require to be polished to give totally flat and planar surfaces for deposition of electronic components onto the surface by standard lithographic and etching techniques to form integrated chip semiconductor devices. Typically a 20 cm diameter wafer will produce forty micro processor chips.
The designed size of such integrated chips is steadily decreasing and the number of layers applied, e.g. by lithography onto the silicon surface is rising, to produce ever smaller and increasingly complex micro-circuits. Present semiconductors typically incorporate 3 or 4 metal layers, whilst it is expected that future designs will contain 5 or more layers. This increase in the number of layers applied is leading to ever more stringent requirements on the smoothness and planarity of the silicon wafers, since pits or scratches may produce voids which cannot be bridged by deposited material, as the widths and thicknesses of deposited layers are decreased, leading to unplanned resistances where a conductor is narrowed, or capacitances/non-conductive gaps, where breaks occur in deposited conductor layers, which interfere with or compromise the planned operation of the circuit.
The standard wafer polishing technique in use at present is to remove a wafer from a stack, or cassette of e.g. 10 wafers, by means of a robot arm, and manoevre the disc into position over a rotating disc. The disc is usually coated with polyurethane, and the wafer is held in place by an overhead platen whilst being polished by the rotating disc. This is an adaptation of optical polishing technology used for polishing lenses, mirrors and other optical components. Once polishing is completed, the robot arm removes the wafer and transfers it to another work station for eventual lithographic deposition steps.
A significantly different approach is so-called Linear Planarization Technology, developed by OnTrak, wherein an endless travelling belt is used to polish the wafer, in place of the rotating disc form of polishing tool. The belt used in this method is described in EP-A-0696495 and comprised an endless belt of sheet steel, having a polyurethane coating of low Shore A hardness. A major problem with these belts is the poor adhesion of polyurethane to steel. An adhesive or coupling agent is required for bonding between the steel and polyurethane to take place but in spite of the use of such an agent bond strength is insufficient to withstand the harsh conditions under which the belt operatesxe2x80x94particularly the frictional forces occurring between the belt and wafer in the zone of contact. The tendency is for the polyurethane to wear out or to flake off within two days or so, and to repair this an area around the damaged coating has to be removed for fresh polyurethane to be added as a patch. This leaves seams or joints between the original coating and the patches which must be removed by complicated and expensive high-precision machinery and processes so as to ensure that a flat planar belt surface is maintained.
An object of the invention is to provide a belt-type apparatus for polishing silicon wafers wherein the problems arising from the use of a sheet metal belt, having a poorly bonded coating, are at least substantially overcome.
This invention provides for use in polishing silicon wafers, an endless belt to act as a polishing tool, characterised in that the belt comprises a woven or non-woven fabric coated with a suitable polymer.
The polymer is preferably polyurethane, preferably with a low Shore D hardness, e.g. from 65-75.
Alternatively the polymer may be any thermoset or thermoplastic polymer having a reasonably high abrasion resistance, such as polyamides, silicones, fluoropolymers, epoxy resins and thermoplastic polyurethanes.
The coating may comprise two or more layers of different hardnesses. The coating may comprise at least one layer of partially fused polymeric particles, or two or more thermoplastic polymers of different melting points.
The upper layer may be the harder layer.
On the other hand the intermediate layer may be the harder layer, and the upper layer may comprise a foamed plastic, or be formed of or incorporate thermally expandable expanded polystyrene beads which form pores in the plastics layer. Hollow microbeads of plastic, glass or soluble material may be incorporated in the upper layer.
Abrasive particles or fibres may be added to the upper layer, which may constitute a transparent coating, or be micro textured with micro-scale grooves or surface roughness.
The fabric may be a substrate which is woven in endless form embodying yarns of high tensile strength and relatively low elongation.
A fabric woven in endless form lacks the weak spots of a seam or splice, which is a great advantage as these belts operate under extremely high tension to prevent the formation of ripples or wrinkles.
The belt thickness is typically 0.1-0.2 inches, whilst the coating thickness is in the range 0.05-0.09 inches.
Examples of suitable yarns are meta- or para-aramids such as KEVLAR, NOMEX OR TWARON; PBO or its derivatives; polyetherimide; polyimide; polyetherketone; PEEK; gel-spun UHMW polyethylene (such as DYNEEMA or SPECTRA); or polybenzimidazole; or other yarns commonly used in high-performance fabrics such as those for making aerospace parts. Mixtures or blends of any two or more yarns may be used, as may glass fibres (preferably sized), carbon or ceramic yarns including basalt or other rock fibres, or mixtures of such mineral fibres with synthetic polymer yarns. Any of the above yarns may be blended with organic yarns such as cotton. The belts according to the invention woven from these yarns are strong in the machine direction and sufficiently rigid in the cross machine direction.
Most preferred are aramid yarns due to their low weight and high strength.
A non woven fabric substrate may be provided in place of a woven substrate and be formed from any one, or a blend or mixture of any of the above mentioned yarns or fibres. More than one nonwoven substrate may be provided, preferably two, and they may be vertically aligned or offset relative to one another.
A belt substrate may comprise a non woven fabric with additional spaced apart linear yarns extending substantially in a common direction, and a polymeric matrix material interconnecting and at least partially encapsulating each of the yarns. The linear yarns preferably are oriented in the running direction of the belt, but may also or instead be oriented in the cross-machine direction, i.e. transversely of the belt e.g as described in GB-A-2202873. Extra reinforcing yarns extending substantially in the machine direction may also be provided.
The belt substrate preferably has a relatively high open area due to the increase in delamination resistance, particularly if the substrate is fully impregnated with polymer. For this, a spiral link belt of the kind disclosed in GB-A-2051154, comprising an array of eg. cross-machine direction hinge wires, connected by interdigitating flattened helical coils is particularly preferred, as one large open area woven fabrics. This substrate may support a woven or non-woven fabric which is coated or partially or fully impregnated with the suitable polymer.
The surface of the belt may be formed with grooves extending in the running direction of the belt to remove wet slurry generated during the polishing process. This slurry can be removed from the belt grooves using one or more high pressure water jets, rotating fine brushes or hard non-metallic (e.g. ceramic) stylii.